Convection with Rotation in a Neutral Ocean: A Study of Open-Ocean Deep Convection

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  • 1 Center for Meteorology and Physical Oceanography, and Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts
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Abstract

The intensity and scale of the geostrophically adjusted end state of the convective overturning of a homogeneous rotating ocean of depth H at a latitude where the Coriolis parameter is f, induced by surface buoyancy loss of magnitude B0, are studied by numerical experiment. The experiments are related to observations and laboratory studies of open-ocean deep convection. A numerical model based on the nonhydrostatic Boussinesq equations is used. The grid spacing of the model is small enough that gross aspects of convective plumes themselves can be resolved, yet the domain of integration is sufficiently large to permit study of the influence of plumes on the large scale and geostrophic adjustment of the convected water.

Numerical simulations suggest that cooling at the sea surface is offset by buoyancy drawn from depth through the agency of convective plumes. These plumes efficiently mix the water column to generate a dense chimney of fluid, which subsequently breaks up through the mechanism of baroclinic instability to form spinning cones of convectively modified water that have a well-defined and predictable scale.

A measure of the importance of rotation on the convective process is provided by a natural Rossby number introduced by Maxworthy and Narimousa:
i1520-0485-23-6-1009-eq1
where lrot = (B0/f3)1/2 is the length scale that marks the transition from three-dimensional, thermally driven turbulence to quasi-two-dimensional, rotationally dominated motions. Here urot=(B0/f)1/2 is the velocity of a particle gyrating in inertia] circles of radius lrot.

In the parameter regime typical of open-ocean deep convection, we find that Ro* ≲ 1; rotation influences the intensity and scale of both plumes and cones. In particular, the scale, intensity, buoyancy excess, and generation rate of the cones of geostrophically adjusted fluid, which result from the breakup of the chimney, are found to depend in a predictable way on this single nondimensional number, formed from the external parameters f, B0, and H.

Abstract

The intensity and scale of the geostrophically adjusted end state of the convective overturning of a homogeneous rotating ocean of depth H at a latitude where the Coriolis parameter is f, induced by surface buoyancy loss of magnitude B0, are studied by numerical experiment. The experiments are related to observations and laboratory studies of open-ocean deep convection. A numerical model based on the nonhydrostatic Boussinesq equations is used. The grid spacing of the model is small enough that gross aspects of convective plumes themselves can be resolved, yet the domain of integration is sufficiently large to permit study of the influence of plumes on the large scale and geostrophic adjustment of the convected water.

Numerical simulations suggest that cooling at the sea surface is offset by buoyancy drawn from depth through the agency of convective plumes. These plumes efficiently mix the water column to generate a dense chimney of fluid, which subsequently breaks up through the mechanism of baroclinic instability to form spinning cones of convectively modified water that have a well-defined and predictable scale.

A measure of the importance of rotation on the convective process is provided by a natural Rossby number introduced by Maxworthy and Narimousa:
i1520-0485-23-6-1009-eq1
where lrot = (B0/f3)1/2 is the length scale that marks the transition from three-dimensional, thermally driven turbulence to quasi-two-dimensional, rotationally dominated motions. Here urot=(B0/f)1/2 is the velocity of a particle gyrating in inertia] circles of radius lrot.

In the parameter regime typical of open-ocean deep convection, we find that Ro* ≲ 1; rotation influences the intensity and scale of both plumes and cones. In particular, the scale, intensity, buoyancy excess, and generation rate of the cones of geostrophically adjusted fluid, which result from the breakup of the chimney, are found to depend in a predictable way on this single nondimensional number, formed from the external parameters f, B0, and H.

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